Conventional alkaline electrochemical cells are primary (non-rechargeable) cells having an anode comprising zinc, a cathode comprising manganese dioxide, and an alkaline electrolyte. The cell is formed of a cylindrical housing. The housing is initially formed with an open end. After the cell contents are introduced, an end cap that forms the negative terminal with insulating plug such as plastic grommet is inserted into the open end. The cell is closed by crimping the housing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. The housing serves as the cathode current collector and a portion of the housing forms the positive terminal.
In general, a primary alkaline cell includes an anode, a cathode, an electrolyte permeable separator between the anode and the cathode, typically containing a cellophane film, and an alkaline electrolyte contacting both the anode and the cathode. The anode includes an anode active material comprising zinc or zinc alloy particles and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte. The gelling agent serves to immobilize the zinc particles in a suspension such that the zinc particles are in contact with one another. An anode current collector, typically a conductive metal nail is inserted into the gelled zinc anode. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but can include aqueous solutions of sodium or lithium hydroxide. The cathode includes a cathode active material comprising manganese dioxide or nickel oxyhydroxide or mixtures thereof and an electrically-conductive additive, such as graphite, to increase electrical conductivity of the cathode.
A common problem associated with the design of primary alkaline cells, zinc/manganese dioxide cells in particular, is the tendency for a cell to generate hydrogen gas when it is discharged below a certain voltage, typically at or near the endpoint of the useful capacity of the cell. Zinc/manganese dioxide cells typically are provided with a rupturable diaphragm or membrane located within the end cap assembly of the cell. Such a rupturable diaphragm or membrane can be formed within a plastic insulating member as described, for example, in U.S. Pat. No. 3,617,386. When internal gas pressure increases to a predetermined value, the membrane can rupture thereby venting the gas to the external environment through apertures in the end cap thereby lowering the internal pressure.
Commercial cylindrical alkaline cells are available typically in AA, AAA, AAAA, C, and D sizes. Since commercial cell sizes and the corresponding internal volumes of these cells are fixed, in order to increase cell capacity, i.e., the useful service life of the cell, it has been necessary to increase the interfacial surface area of the electrode active material as well as to include greater amounts of active material in the cell. This approach has several practical limitations. If the active material is packed too densely into the cell this can produce a decrease in the rate of electrochemical reaction during discharge, thereby reducing service life of the cell. Other deleterious effects such as polarization can occur, particularly at high current drains (i.e., in high power applications). Polarization limits mobility of ions within the electrode active material as well as within the electrolyte, thereby reducing service life of the cell. Contact resistance between the cathode active material and the cell housing also can reduce service life.
Another problem associated with a zinc/manganese dioxide primary alkaline cell is that the cell characteristically has a sloping voltage profile, that is, the average running voltage gradually decreases as the cell is discharged. The rate of decrease in voltage is more pronounced as the cell is discharged at higher power drain rates, for example, either constantly or intermittently between about 0.25 and 1 Watt (i.e., between about 0.3 and 1 Amp), particularly between about 0.5 and 1 Watt. Thus, for a zinc/manganese dioxide cell, the actual cell capacity (milli-Amp-hrs) obtained at high power drain rates can be substantially less than at low power drains.
Thus, there is a need for a primary alkaline cell better suited to high power applications. Such a cell could be used as the main power source for a high power device or as a back-up power source to supplement a rechargeable battery to power such devices. Modern electronic devices such as cellular phones, digital cameras, digital audio players, CD/DVD players, handheld televisions, electronic flash units, remote controlled toys, personal digital assistants (i.e., PDAs), camcorders and high-intensity lamps are examples of high power applications. Thus, it is desirable to provide an improved primary alkaline cell having longer service life than a conventional zinc/manganese dioxide alkaline cell of the same size, particularly for use in those applications demanding high power.
Accordingly, it is desirable to provide such an improved alkaline cell in order to extend the useful service life of primary alkaline cells intended for use in high power devices.
It is also desirable to provide an improved alkaline cell having a reduced amount of hydrogen gassing, thereby improving storage characteristics and simplifying requirements for a suitable venting system.